1. Field of the Invention
The present invention relates to a wave receiving apparatus for receiving and processing waves, such as acoustic waves (including ultrasounds and vibration), and electromagnetic waves, and an ultrasonic diagnostic apparatus which is one application field of the wave receiving apparatus.
2. Description of the Related Art
Hitherto, a wave receiving apparatus for receiving and processing waves has been generally adopted. An ultrasonic diagnostic apparatus is one application field of the wave receiving apparatus. As other examples of the application, there are widely known a radar for detecting the position of airplanes or the like in the air, a fish detector for detecting the position of fishes in the water, a marine seismic profiling system, and a flaw detector for detecting flaws inside an object.
Here, an ultrasonic diagnostic apparatus is referred to, by way of example, from among those various applications. The earlier technology thereof will be described hereinafter.
The ultrasonic diagnostic apparatus is well known as a system in which mainly a human body is offered as the subject. A process in which ultrasound is transmitted to the inside of the subject and ultrasounds reflected by the surfaces of various tissues inside the subject are received is sequentially repeated to scan the inside of the subject with ultrasounds. By this process, an image inside the subject is displayed on the basis of received signals derived through the scanning process, thereby contributing to diagnosis of illness such as diseases of the viscus inner organs.
FIGS. 19(a)-(c) are illustrations useful for understanding a state in which an ultrasonic diagnostic apparatus is used to obtain an image of a target (an ultrasound reflector) within the subject.
The ultrasonic diagnostic apparatus is provided with, as shown in FIG. 19(a), a large number of ultrasonic transducers 1 arranged in a predetermined direction (horizontal direction of FIG. 19(a)). These ultrasonic transducers 1 are applied to a body surface of the subject to transmit ultrasonic pulses toward the inside of the subject. The pulses are transmitted by means of driving with electric pulses a plurality of ultrasonic transducers included in a certain aperture 2 set up for receiving ultrasounds of one timing. An ultrasonic beam 4 having a directivity is formed within the subject in such a manner that when ultrasounds are transmitted, timings for driving the plurality of ultrasonic transducers included in the aperture 2 are controlled. In addition a drive intensity for driving each of the plurality of ultrasonic transducers is controlled in accordance with a predetermined weighting function 3. In the weighting function 3, an arrangement position (an arrangement sequence) of the plurality of ultrasonic transducers included in the aperture 2 is given in the form of a variable.
Ultrasounds reflected within the subject and returned are received by the plurality of ultrasonic transducers constituting the aperture 2, respectively. The received signals are amplified in accordance with the associated weighting functions 3, respectively, while beamformed so as to emphasize the ultrasonic reflection signal in the direction along the ultrasonic beam 4 extending into the inside of the subject. This is referred to as "the received beam is formed". On the other hand, the ultrasonic beam transmitted to the inside of the subject is referred to as the transmitted beam. The beamforming process is referred to as a phasing addition and is a well known technology. Thus, in this respect, a redundant description will be omitted.
Such a process of transmission and reception for ultrasounds is repeatedly performed while the aperture 2 is sequentially shifted in a direction of an arrangement of the ultrasonic transducers 1. A process such that while the aperture 2 is sequentially shifted, the process of transmission and reception for ultrasounds is repeatedly performed, is referred to as a scan.
It is noted that, for the purpose of simplification of the explanation, the above explanation has been made without especial distinctions between the transmission aperture and the reception aperture; the weighting function for transmission and the weighting function for reception; and the transmitted beam and the received beam. However, it is acceptable that they are different from one another between the transmitting end and the receiving end. They may be suitably set up in the transmitting end and the receiving end, respectively.
It is possible to obtain images within the subject by means of displaying the intensity of the signals representative of a plurality of received beams, which are obtained through the above-mentioned scan process, in the form of luminance. Now consider a case where only one target exists within the subject. In view of the fact that the ultrasonic beam 4 (both the received beam and the transmitted beam) has a directivity, the intensity of the received signal on each of the apertures set up in the scan process offers the respective value as shown in FIG. 19(b). The distribution of intensity of those signals is referred to as a beam profile.
FIG. 19(c) shows an image (a target image) in which the received signals having such a signal intensity distribution are represented by a luminance.
While a resolution of the ultrasonic diagnostic apparatus is better with smaller target images, usually, a size of the target image is significantly expanded as compared with the target 5 itself.
Hitherto, the distribution of intensity of received signals, which determines a size of the target image, that is, the beam profile, is determined in accordance with a size of the aperture 2, the weighting function 3 and a wavelength x of the ultrasounds to be transmitted and received. Also, it has been devised that those elements are set optimally. However, there is a limit in improvement of the resolution.
Further, in a case where a position of the target 5 is determined, a position of the target 5 as to the arrangement direction of the ultrasonic transducers 1 only can be determined, when the subject is scanned to determine the peak of intensity of the received signals. For example, it may happen that the ultrasonic beam 4 is biased with respect to the target 5 as shown in FIG. 19(a). In such a case, even if a sufficient signal intensity of received signals is obtained through the reflected ultrasounds from the target 5, it has been impossible to determine a displacement or a direction (an angle) of the target 5 through one ultrasonic transmission and reception process.
Furthermore, with respect to the detection of the position of the target, intervals of the apertures, are sequentially set up when scanned. If these intervals are rough, so that the apertures are set up merely at intervals with respect to the horizontal direction of FIG. 19(a) for instance, it is impossible to detect the true peak. This involves a decline in the detection accuracy for a position of the target. On the other hand, intervals of the apertures may be set up to be fine in order to attain a sufficient accuracy in the position detection of the target, In this case, there is a need to perform ultrasonic transmission and reception processes a large number of times by the number of times corresponding to a number of apertures set up on a fine basis. Thus, it takes a lot of time for scanning the subject once. This involves a decline in the frame rate.
While the above explanation has been made exemplarily for the ultrasonic diagnostic apparatus, the above-mentioned problems are not ones involved in only the ultrasonic diagnostic apparatus. Rather, these are common problems for all of the apparatuses in which waves are received to detect a position or the like of the target. Further, the above-mentioned problems are common problems for not only a case where a target, which reflects waves, is offered as the object, but also an apparatus for detecting a position of a target which generates per se waves such as acoustic wave, electromagnetic wave and the like.